1. Field of the Invention
The present invention relates to a biaxially oriented, unidirectionally long polyethylene-2,6-naphthalenedicarboxylate film. More specifically, it relates to a biaxially oriented, unidirectionally long polyethylene-2,6-naphthalenedicarboxylate film useful as a substrate film for a magnetic recording medium for high-density recording, which is capable of recording for a long period of time and excellent in electromagnetic conversion characteristics, particularly a magnetic recording medium having a thin magnetic metal film formed by vapor deposition such as a magnetic recording tape for use with VTR.
2. Prior Art
As a magnetic recording film which has high coercive force for magnetic recording and is capable of recording for a long period of time, there has been proposed a magnetic recording tape which is obtained by preparing, as a substrate, a biaxially oriented polyethylene-2,6-naphthalenedicarboxylate film having specific mechanical properties and a specific surface structure, forming a magnetic layer of a thin metal film on one surface of the substrate and forming a coating of an organic polymer containing a lubricant on the other surface of the substrate where no thin magnetic metal film is formed. This magnetic recording film has an improved coercive force for magnetic recording due to the use of a polyethylene-2,6-naphthalenedicarboxylate film having a stress of at least 18 kg/mm.sup.2 when elongated in the longitudinal direction by 5%, a Young's modulus of at least 800 kg/mm.sup.2 in the longitudinal direction and a Young's modulus of at least 500 kg/mm.sup.2 in the transverse direction.
However, the above film still involves problems in that the substrate film undergoes curling when a thin metal film is formed thereon by vapor deposition and causes a trouble in running at a step of forming a magnetic recording tape, that the tape fails to give an adequate output or the tape runs unstably since it has curling and falls to be in good contact with a magnetic head when used after the tape has been taken up and encased in a cassette.